In the field of ophthalmic lenses, particularly contact lenses, numbers for showing the specifications, etc., are printed on a part of the lens in order to identify specifications such as dioptric powers of contact lenses to manage during their production, distribution and sales. In addition, for easy distinction between the front and back surfaces of the lens, numbers, characters, figures, and the like are similarly printed for indication. Furthermore, in light of esthetic preference, a contact lens colored to provide an iris pattern may be used for patients who have defected iris of an eye due to a disease, injury or the like such that a natural appearance is achieved by printing a contact lens to imitate colors and shapes of the iris of human eyes. Alternatively, in light of fashion-oriented aspects, contact lenses having varying iris color are also highly demanded. Thus subjecting an ophthalmic lens to marking or dyeing is an important feature in producing such a lens.
Conventionally, known methods for printing characters, numbers, figures and the like on an ophthalmic lens include a pad printing method and a screen printing methods in which a dye composition for an ophthalmic lens containing a dye or a pigment is used, and the like. In the pad printing method, an ink solution is retained in recessed grooves formed by engraving characters and the like to be marked, and the ink solution is transferred to a pad, which pad carrying the ink is then brought into contact with a substrate to execute printing. In the screen printing method, a mesh is laid over holes having the shapes of the characters, numbers, figures, etc., and then the mesh is bonded with compression on the substrate, followed by applying a dye composition for an ophthalmic lens from above thereof to execute printing to provide desired shapes at specific sites. However, these methods are accompanied by disadvantages as in the following. For example, when marks are printed on desired positions of an ophthalmic lens such as a contact lens by the pad printing method, it is necessary to provide grooves corresponding to the shapes of the numbers and characters to be printed by etching on the base such as a metal beforehand. Such etching requires a considerable time period, and thus making stable supply of the base difficult. In addition, according to the screen printing method, it is necessary to produce holes having the shapes of the numbers, character, etc., to be printed beforehand using an emulsion or the like, which process also necessitates a considerable long time period similarly. Moreover, when a contact lens is actually printed by, for example, these methods, compression bonding of the pad or screen to the contact lens is necessary so as not to be movable; however, this operation requires a fixation device or jig for exclusive use for permitting the compression bonding. In the case of the screen printing method, for example, it is necessary to fix the screen while pulling it to complete compression bonding of the same on the contact lens, and thus a large device is required. Moreover, it is necessary to provide screens for dyeing and etching bases for each different character, number, and figure. In addition, since the device must be repeatedly assembled every printing step of each contact lens, or every batchwise printing step, leading to involvement of a very complicated step. Therefore, according to the pad printing method or the screen printing method, the number which can be processed is limited resulting from necessity for adjustment of the device and complexity of production steps for the method, thereby leading to increase in the cost, as well as needs for additional labor for control and management in connection with the device and the steps, and the like. Furthermore, the variance of the amount of applied ink, and the variance of the compression bonding properties of the ink, may result in poor transfer of the ink, or sometimes may lead to blurring of the printing, or deterioration of sharpness of the printing.
Thus, in place of the pad printing method and screen printing method, a method for marking with an ink-jet coating equipment has been proposed. An ink-jet coating equipment applies a dyeing solution in the form of extremely fine droplets on the face to be dyed, and the droplet attached on the surface form one dot. Thus, assembly of the dots enables a pattern having a desired character, number or figures to be formed. Generally, a character, number, or figure when formed with small dots assembled with a high density gives a sharp contrast to provide a mark that is superior in visibility. When printing or drawing is carried out using an ink-jet coating equipment, the size of the droplet, and the point where the droplet attaches, and the like can be accurately controlled by a computer or the like. The shape and the size of the mark can be easily changed by alteration, etc., of the computer program. In addition, it is also possible to print a plurality of dyes at the same time by using a plurality of ink jet nozzles, whereby superior characteristic feature is achieved in that various color formation and the like can be easily conducted, accompanied by excellent color contrast. Therefore, in printing using an ink-jet coating equipment, it is not necessary to provide a mold or screen engraved for different characters, numbers, figures, respectively, as in the pad printing method and the screen printing method. Moreover, the printing with an ink-jet coating equipment is based on a method of a noncontact type and does not require an extra step for allowing the pad or screen to be contacted or compression bonded; therefore, a superior advantage enabling to realize printing with very high productivity, and with a constantly high quality is achieved. In view of such advantages of the ink-jet coating equipment, some methods for marking an ophthalmic lens such as a contact lens have been disclosed.
Japanese Unexamined Patent Application, Publication No. H8-112566 discloses a method for printing a dye composition for an ophthalmic lens on a desired part of a hydrous contact lens with an ink-jet coating equipment. This document shows that use of an ink-jet coating equipment enables marking that is superior in workability is enabled. However, the document discloses use of a reactive dye, a pad dye or the like as the dye composition for an ophthalmic lens. When marking or dyeing is carried out on an ophthalmic lens such as a contact lens, it is necessary to prevent elution of the dye from the marking site in light of securement of the safety in eyes since these ophthalmic lenses are used by bringing into direct contact with the eyes. Furthermore, contact lenses, particularly hydrous soft contact lenses are swollen by immersion in water during storage, or during use, and as needed must be subjected to a treatment such as boiling or washing. It is necessary to prevent elution of the dye from the marking site in light of the qualitative durability, even if the contact lenses are thus swollen with water, or subjected to a treatment such as boiling or washing multiple times. However, the reactive dyes among the dyes used according to the prior art described above must have an active group, which can react with the dye, in the chemical structure of the material of the ophthalmic lens, and the dye may be eluted outside due to lack of the reaction of the dye in the case of the ophthalmic lens material not having an active group that can react with such a reactive dye. Therefore, the reactive dye disclosed in the prior art is disadvantageous in limitation of the type of applicable ophthalmic lens materials. On the other hand, when a pad dye is used, it is usually converted into a reduced state by making an alkali solution to prepare a dye solution; however, such a solution is readily oxidized in the air to be an insoluble substance. Therefore, the dye may be hardened within a discharge nozzle of an ink-jet coating equipment, whereby clogging may be caused. In order to prevent such events, an operation under an inert environment is required, and thus apparatuses and facilities enabling production or packing of an ink and application by an ink jet system in an inert environment would be necessary, leading to involvement of disadvantages in production such as necessity of significant costs. Additionally, there exist problems of economical efficiency and environmental aspects since an alkali solution and a fixing solution must be used in the case of the pad dye, due to a considerable expense required for a treatment of unwanted solutions (waste product) after the operation, and the like.
A dye composition has not been developed which is hardly accompanied by elution of the dye even in an ophthalmic lens in which its material does not have a group that can react with the reactive dye, when the lens is produced by a method for producing a colored ophthalmic lens using an ink-jet coating equipment. Furthermore, there is no economically and environmentally conscious method for producing a colored ophthalmic lens which can avoid use of a large quantity of an alkali solution and fixing solution, under current circumstances.